1. Background
Historically, cable television (CATV) infrastructure has been optimized for providing broadcast services, such as broadcast television. Conventional CATV infrastructure includes hybrid-fiber coaxial (HFC) architecture that provides low cost distribution of broadcast services.
CATV providers, no longer offering only broadcast television, now offer a wide range of services, with new broadband services constantly being introduced. These new services demand more Mbps/user, and eventually the demand may exceed network capacity.
As the price of optical components drops and network demand increases, HFC architectures may cease to be the most efficient infrastructure for delivering broadband services to the home. It is well known that fiber-optic access systems (i.e., systems that bring fiber closer to the home than conventional HFC architecture, such as fiber-to-the-home (FTTH), fiber-to-the-curb (FTTC) and the like) can have far greater capacity than HFC architectures. However, to displace conventional HFC systems, a new architecture including fiber-optic access systems is needed for delivering broadband services.
Migration to an architecture including a fiber-optic access system cannot be accomplished overnight. Thus, for a prolonged period, CATV infrastructure will include both fiber-optic access systems and HFC architecture. Therefore, there is a need for a CATV infrastructure employing fiber-optic access systems and HFC distribution plants that share a primary hub and other equipment.
Additionally, conventional HFC architectures generally utilize a modulation format having a high signal-to-noise ratio (SNR) that is required for broadcast transmissions. For example, traditional, analog, broadcast-television signals employ amplitude modulation vestigial sideband (AM-VSB), which requires a very high carrier-to-noise ratio (CNR), and traditional HFC architectures have been designed to support this.
Also, because coaxial cable has limited bandwidth, bandwidth efficient modulation formats are used. For example, downstream digital signals (e.g., digital video or data) typically employ quadrature-amplitude-modulation (QAM), such as 64-QAM or 256-QAM, with either 6 or 8 bits per symbol. Channels are generally separated by 6 MHz, and consequently, a 5 Msymbol/sec data rate is employed. A simple calculation shows that if the full 55 MHz–860 MHz band is used to carry digital signals, the coaxial cable can carry over 5 Gbps (i.e., 134 channels×5 Msamples/channel×8 bits/sample for 256-QAM). This is shared by all the homes on the coaxial bus, which can vary between 50 and 2000 homes.
It is desirable to use fiber-optic access systems, such as FTTH, in distribution architectures, because the bandwidth of fiber-optic links is much greater than coaxial cable. However, it can be expensive to provide fiber-optic links capable of maintaining the necessary SNR for conventional CATV modulation formats, such as QAM and AM-VSB. A low-noise optical link employing a spectrally efficient modulation format generally requires that the link be operated with a high-power receiver. Also, higher SNR requirements imply a lower tolerance for impairments in optical links caused by optical fiber non-linearity. These two conditions (i.e., more power at the receiver and low tolerance for optical fiber non-linearity) reduce the maximum span length between optical amplifiers, which increases system costs. Therefore, a need exists to provide a distribution system employing fiber-optic access systems using a robust modulation format for downstream transmission of information and having a relatively inexpensive cost of implementation. Additionally, conventional customer premises equipment (CPE), such as a television set, set-top box, cable modem, digital/analogue telephone, set-top Internet access device, personal digital assistant and any device configured to receive signals via a CATV infrastructure maybe configured to receive information in conventional CATV modulation formats (e.g., AM-VSB and QAM). Therefore, a need exists to provide a distribution system employing fiber-optic access systems that supports conventional CATV modulation formats.
2. Prior Art
Woodward et al. discusses in a 1996 IEEE publication entitled “A Passive-Optical Network Employing Upconverted 16-CAP Signals” that an FTTC PON employing a 16-CAP modulation format transmits information downstream to optical network units (ONUs) in close proximity to users' homes. However, Woodward et al. does not disclose converting signals in a 16-CAP format to a modulation format compatible with CPE.
Wilson et al. discusses in a publication entitled “Reduction Of Optical-Beat Interference (OBI) in Cable-Modem/FTTH Systems Using Burst-Mode Lasers” that a FTTH architecture transmits information downstream using a 64-QAM modulation format. However, transmitting information downstream over a fiber-optic link using 64-QAM generally requires high power at the receiver and a low tolerance for impairments in optical links. These two conditions (i.e., more power at the receiver and low tolerance for fiber non-linearity) reduce the maximum span length between optical amplifiers, which increases system costs.